1. Technical Field
The present disclosure relates to an orthogonal frequency division multiplexing (OFDM) receiver, and more particularly, to a method and apparatus for estimating channel state information to detect noise and an interference signal included in frames transmitted by an OFDM transmitter, and a receiver having the apparatus.
2. Discussion of Related Art
An orthogonal frequency division multiplexing method (hereinafter, referred to as “OFDM”) is a wide band modulation method for dividing a frequency band width assigned for a communication session into a plurality of narrow band frequency sub-bands. Each of the narrow band frequency sub-bands includes a radio frequency (RF) sub-carrier. Each sub-carrier is mathematically orthogonal to the RF sub-carrier included in each of the other sub-channels.
The OFDM method is a multi-carrier modulation method in which data to be transmitted is primarily converted to a complex symbol in the form of M-ary QAM (quadrature amplitude modulation). A complex symbol sequence or a series of complex symbols is converted to a plurality of parallel complex symbols through a serial-to-parallel conversion. Each of the parallel complex symbols is rectangular pulse-shaped and sub-carrier modulated.
In the multi-carrier modulation method, the frequency interval between the sub-carrier is set such that all sub-carrier modulated parallel complex symbols are orthogonal. Thus, in the OFDM method, spectrums of the sub-carriers are overlapped with one another without interruption by other carriers due to the orthogonality of the sub-carriers. Since the frequency bandwidth is divided into a plurality of orthogonal sub-bands, a high data transfer speed and an efficient use of a bandwidth may be possible.
Each of the sub-carriers used in the OFDM method may have independent channel information. Thus, each sub-carrier may have a different signal to noise (S/N) ratio according to a fading property of the channel. For example, sub-carriers in a multi-path channel may have different S/N ratios, since the data loss of a sub-carrier having a null section is greater than that of a sub-carrier without the null section. A null section may be caused by white noise.
In addition, data of each sub-carrier can be distorted when a frequency selective interference signal is input to a co-channel. Examples of frequency selective interference signals include a national television system committee (NTSC) signal, a phase alternation by line (PAL) signal, or an FM modulated signal. Examples of co-channels include an ultra high frequency (UHF) channel or a very high frequency (VHF) channel.
A proposed method reduces co-channel interference to reduce the distortion of data. A degree of noise due to fading or interference by an interference signal is measured, the measured values are output as channel state information (CSI), and information of the sub-carrier is decoded based on the CSI. Signal power and noise power are calculated after the channel response is estimated using pilot information of a sub-carrier output from an equalizer. However, the structure of the equalizer may be complex and transmission performance may deteriorate due to input noise of a receiving end. In addition, it may be difficult to apply the method to a communication system that does not include the pilot information or the equalizer, for example, digital audio broadcasting (DAB) or terrestrial-digital multimedia broadcasting (T-DMB).
Thus, there is a need for a method and apparatus for estimating channel state information which can detect noise and an interference signal included in transmitted frames based on the average powers of null symbols of the transmitted frames output from an OFDM transmitter.